Smooth high tolerance porous tube and process for making

ABSTRACT

SEVERAL BENEFICIAL PROPERTIES ARE ACHIEVED BY CENTRIPETALLY MECHANICALLY WORKING AS BY ROTARY SWAGING OF POROUS METAL TUBING PARTICULARLY PREPARED FROM SINTERED POWDERED METALS. THESE INCLDE EXCELLENT DIMENSIONAL TOLERANCE, EXCELLENT SURFACE FINISH, CONTROLLED DENSIFICATION DEPENDING ON THE AMOUNT OF REDUCTION, INCREASED STRENGHT AND YET THE OVERALL POROSITY OF THE TUBE IS RETAINED TO A VERY CONSIDERABLE EXTENT. THE INTERNAL SURFACE RETAINS ITS INITIAL ROUGH STRUCTURE.

United States Patent 3,700,419 SMOOTH HIGH TOLERANCE POROUS TUBE ANDPROCESS FOR MAKING Frederick J. Sorgenfrei, Lake Elmo, Minn., assignorto Minnesota Mining and Manufacturing Company, St. Paul, Minn.

No Drawing. Original application Aug. 28, 1969, Ser. No. 853,972.Divided and this application Jan. 25, 1971, Ser. No. 109,696

Int. Cl. B22f 1/00 U.S. Cl. 29182.2 3 Claims ABSTRACT OF THE DISCLOSURESeveral beneficial properties are achieved by centripetally mechanicallyworking as by rotary swaging of porous metal tubing particularlyprepared from sintered powdered metals. These include excellentdimensional tolerance, excellent surface finish, controlleddensification depending on the amount of reduction, increased strengthand yet the overall porosity of the tube is retained to a veryconsiderable extent. The internal surface retains its initial roughstructure.

This is a division of application Ser. No. 853,972, filed Aug. 28, 1969,now Pat. No. 3,626,744.

This invention relates to porous metallic tubes and particularly toporous metallic tubes having highly finished metallic outer surfaces,and sintered internal surfaces.

It is generally not possible to produce smooth accurately dimensioneduniform porosity tubing by normal powder metallurgy techniques. Shortlengths of smooth tubing can be produced by die pressing and sintering.It is not possible, however, to produce long lengths of tube by diepressing and other powder metal techniques must be used. The sinteredsurface of materials made by these processes are usually very rough andit is diflicult to hold machine tolerances. There is also difliculty inthat they tend to smear and thereby lose surface porosity when machinedas a result of the tangential application of forces. This problembecomes increasingly more diflicult as the porosity (micronic rating) ofthe tube decrease. Several techniques have been proposed to facilitatethe machining of porous materials without smearing the surface. All ofthese involve filling of the pores with a substance that can be removedafter machining. For example, certain salts may be used to impregnateporous materials and, after machining or grinding is performed in theimpregnated condition, the impregnant is removed. These are cumbersomehigh cost and multiple step processes and are very diificult to control,especially for long tubes. The present invention has as an object tofinish and dimension the outer surface of porous sintered powdered metaltubing while maintaining uniform porosity to a substantial proportion ofthat of the unfinished tubing. The process of the invention provides avery inexpensive simple method of achieving these desirable results. Theinner surface remains grainy or rough and the outer is dimensioned andfinished to an extent determined by the extent to which the outerdiameter is reduced.

The prior art shows that porous metallic tubes can be made and it hasbeen suggested by Mott, U.S. Pat. No. 3,313,621, that by insertion of asuitably shaped mandrel the tube can be hammered or swaged on themandrel to shape the inside and outside. This process tends to reduceporosity quite sharply because both the outer surface and inner surfaceare subject to deformation. The mandrel in effect exerts a centrifugalforce in opposition to the hammering.

It has now been found that centripetally mechanically working the outersurface simultaneously in two to three ice or more longitudinal zonessimultaneously, as illustrated particularly by rotary swaging, isadvantageously performed on porous sintered metallic tubing formed frommetal powders until the diameter has been reduced to a desired extent.No mandrel is used. Unexpectedly the porosity is not greatly reducedwhen the outer or inner diameter is reduced by up to about 20-50%. Thismay be because there is no outward or centrifugal deformation of theinner surface. The increase in length usually associated with swagingoperations is minimized by the process of the invention. Furthermorereduction in wall thickness is much lower than when a mandrel isemployed.

The tubes used in this invention are preferably made from sinteredpowdered metals using conventional powder metallurgy techniques.Centripetal mechanical forming is then used on the outer surfaces at 2to 4 positions or zones without applying opposite or centrifugal forcedirectly to the inner surfaces. Thus swaging using rotary swagingprocedures applies centripetal force and avoidance of a mandrel omits acentrifugal force. An excellent description of this metallurgicalprocess (without respect to an internal mandrel) is found in Review ofthe Powder Metallurgy Process, July 1966, published by the U.S. ArmyProduction Equipment Agency, Manufacturing Technology Division, RockIsland Arsenal, Ill. Also see Mott, U.S. Pats. Nos. 2,792,302 and3,313,621. Rotary swaging is described in Metals Handbook, T. Lyman,editor 8th ed. (1969), volume 4, pp. 333 et seq.

The preferred powdered metals used in this invention are alloys such asaustenitic chromium-nickel stainless steel. These alloys generallycontaining 16.0 to 26.0 weight percent chromium, 6.0 to 22.0 weightpercent nickel, 0.03 to 0.25 weight percent carbon, and occasionallysome other elements are added to develop certain specific properties,such as 1.75 to 4.00 weight percent molybdenum or small amounts oftitanium, tantalum, and niobium to minimize formation of chromiumcarbides, especially in welding. Standard types of these steels havebeen assigned numbers and specifications by the American Iron and SteelInstitute. These are generally known in the art as stainless steels ofthe AISI series, types 301, 302, 304 and 305 generally referred to as18-8 stainless steel, and the workhorse type 316 generally referred toas 18-8 Mo. All of these AISI stainless steels of the 300 series areapplicable in the practice of this invention. Of course, other ductileor malleable powdered metals can be used in fabricating the tubes usedin this invention, such as nickel, iron, cobalt, copper, and the like,and alloys of such metals, including bronze, monel, etc.

Filters are made from powdered metal which may vary widely in coarsenessfrom as low as 20 or 35 microns up to about 1 mm. selected so that, uponsintering the resulting shaped article, the desired permeability,porosity or micronic rating is obtained. For purposes of making filterelements, it is preferred to use mesh sizes in the range of 20 +325(40-800 microns), such as -200 +325 (40-72 microns), +200 (92 tomicrons)- --50 +100 (150 to 300 microns), 20 +50 (300 to 800 microns) orblends thereof, suitably selected to produce the desired micronic ratingor bubble point, and to that end small amounts, e.g., 1-20 weightpercent, or +325 mesh 40 microns) or even -400 mesh 30 microns) powderedmetals are blended with the coarse powder, i.e., with the 50 +325 mesh(40-300 microns). The term rnes referred to herein means mesh sizeaccording to U.S. Standard Sieve. Approximate closest sizes in micronsare indicated parenthetically. The use of powdered metal with these meshranges will enable one to make tube structures which can be swaged inaccordance with this invention with various micronic ratings, e.g.,maximum beads passed in the range of 1 to 150 microns.

In fabricating each of the filter component layers, the powdered metalof desired mesh is blended with an organic heat-fugitive binder, such asthose disclosed in US. Pats. Nos. 2,593,943 of Wainer; 2,709,651 ofGurnick et al.; and 2,902,363 of Joyner; the preferred binder is methylcellulose with which the lubricants used by Mott in US. Pat. No.2,792,302 are unnecessary. Various solvents can be used in conjunctionwith these binders, such as water, as Well as various plasticizers, suchas glycerin. The blending can be carried out in a conventional manner invarious types of commercially available mixers, blenders, tumblers, andthe like, care being taken to insure that the blend is homogeneous andthe components well dispersed. The resulting blend will be in the natureof a plastic mass or dough and will be similar in consistency to that ofmodeling clay. It is extruded by conventional methods.

Sintering atmosphere, temperature, and duration of sintering dependsupon the particular powdered metals used and the selection of theseconditions is within the skill of the art. In the case of the austeniticstainless steels mentioned above, a hydrogen or dissociated ammoniaatmosphere with a dew point of -40 F. or lower and sinteringtemperatures in the range of 1200 to 1400 C., preferably 1250 to 1350C., is suitable, and the duration of sintering is usually from 10minutes to 2 or 3 hours.

As is evident from the above, the porous tube is made entirely frompowdered metals without requiring or emloying wrought metal componentsor welding. Swaging is carried out on a rotary swaging machine ofconventional type for example, the 2 die type illustrated in FIG. 4,page 334 or a 4 die type illustrated in FIG. 7 page 335 of the aboveMetals Handbook Article. In general the swaging operation is used as afinishing operation to provide close outside diameters as well as forthe usual purpose of decreasing sizes. The surprising feature is that inthis operation it is found that no internal mandrel is desirable andwall thickness is not greatly affected. Also with moderate amounts ofswaging or percentage decrease in diameter, porosity is decreased tomuch less extent than when a mandrel is used and there appears to be notendency for partial plugging of pores so that additional etching stepsare not needed.

The desired surface finish and porosity are produced by suitablecombinations of mesh size of the starting material, green forming andsintering parameters and the amount of reduction during swaging. Theformation of the initial tubes is not part of this invention and tubeshaving calibrated porosities (bubble points) are obtained directly. Anexample of formation is included solely for convenience to readershereof.

The final size and shape of the tube is determined by the size of theswaging die. Various shapes are illustrated in FIG. 8 of the MetalsHandbook Article so that tapers, contours or points may be introduced ifdesired. Single or multiple reductions can be made with or without anintermediate annealing step if desired. All these will be within theskill of the art from the present disclosure.

The articles produced by the process of the invention, i.e., highsurface quality, close tolerance, porous tubes have many applications,for example, as frictionless air turns, as film de-curling bars, as webor film coaters, as air clamps, etc. The tubes can be used for airbearings, e.g., for handling yarns or textiles or for applying lubricantto yarn after spinning. The lubricant can be forced through the poroustube and applied to the yarn. The smooth surface of the porous tubeavoids damage to the yarn. Other applications are in places where lowfriction tube or rod sliding is involved, filters having fine micronicratings microns absolute), flow controllers, flow restrictors anddiffusers.

Although the practice of this invention is described with respect tostainless steel, it is applicable to any porous malleable material suchas the copper based alloys, especially brass and bronze. Otherapplicable metals are nickel and nickel alloys, especially superalloysas cupronickels and monels, cobalt and alloys thereof, other iron alloysincluding low alloy steels, precipitation hardening stainless steels andferrous superalloys. Ductile reactive metals and alloys can also be usedsuch as titanium, zirconium, niobium, tantalum and their alloys. Swagingof Group VI metals is difiicult and must usually be done above roomtemperature. Aluminum is difficult to sinter, especially into tubularconfigurations but products made of it and its alloys can also beimproved by this invention.

Porosity of tubing such as here described may be measured by ASTM TestE128-61 or it may be estimated as to the largest pores by the BubblePoint test described in the report of Micro Metallic Corp., Developmentof Filters for 400 F. and 600 F. Aircraft Hydraulic Systems, WADC TR56-249. Pressure drop across the porous surface measured in suitableunits at various rates of flow is subject to the difficulty that thecapacity of a long porous tube may not be reached at feasible flowrates.

Tubes may be open at both ends if desired or closed at one end. Thefollowing shows how such a tube may be made to be swaged in the processof the invention. Preferably porosities will be in the range of fromabout 1 to microns with pressure drops less than 50 cm. of mercury.

A clay-like mass is produced by first dry-blending 3.0 kg. of 316 Lstainless steel powder of 100 to 200 mesh (92 to 150 microns) size and150 grams of methyl cellulose and then blending with 600 cc. of 10% byvolume glycerine in water for about 1 hour in suitable apparatus such asa Braeblender Sigmablade mixer. The clay-like mixture is extruded byconventional techniques using standard dies for the purpose. A suitableapparatus is a ton Loomis extrusion press. Pieces are extruded up toabout 1.2 meters (4 feet) in length having outside diameters of about0.51 inch (13 mm.) and internal diameters of about 0.31 inch (7.7 mm.).The extruded pieces are air-dried for 12-15 hours and prefired at 2150F. (1170 C.) for two hours in dissociated ammonia. A second quantity ofthe clay-like mass is extruded through a .327 inch (8.3 mm.) die as arod. The rod is dried overnight and prefired at 2150 F. (1170 C.) fortwo hours. The rod or plug is isopressed at 35,000 psi. and is theninserted into one end of the tube. The assembled structure with suitablesteel mandrel as a filler to prevent collapse is similarly isopressed,the mandrel is withdrawn and the tube is then sintered for two hours at2460" F. (1350 C.) in a dissociated ammonia atmosphere. Variations insizes of particles together with conditions of pressing and firing givetubes having various porosities.

Tubes of stainless steel having various porosities and lengths arereduced from about 0.410 inch (10.8 mm.) outer diameter to 0.376 inch(9.53 mm.) outer diameter using a rotary swaging machine to providecentripetal mechanical working and inch (-9.5 mm.) long dies with apartial taper. Similar results are attained using rotary swages withfour dies affecting different longitudinal zones. The finished tubes arecharacterized by micronic rating and bubble point determined asdescribed in the above-mentioned ASTM procedure and WADC report.Accurate measurements of internal and external diameters, are made andair is forced through the tubes at various rates and pressure drops aremeasured in centimeters of water or mercury depending on relative areasof flow involved. The data are summarized in Table 1. In tubes of thelengths and porosities of Examples III and VI closed at one end it isfound that about 98% of the flow occurs in the proximal 12 inches andabout 84% in the proximal 6 inches. The tubes of all examples with thepossible exception of Example I may therefore be considered as being ofapproximately 12 inch effective length in the pressure drop tests. Therelatively rough internal surfaces are characteristics of tubes preparedby this process. A deburring operation may be employed when ends havebeen cut.

TABLE I Micronic rating from Pressure drop (cm. H Percent bubble pointInner Outer 811' flow" of theo- (p) (B-before) diameter diameter reticalEx. (A-after) (inch) (inch) 40 81 121 162 density B 4.8 l .210 421 '3. 8'8. 12. 7 '16. 6 74 A 3.0 .205 376 21.6 "37.5 '43 '46 80 B 7.6 5 .237402 3.7 9.6 12 7 16.7 63

6.5 .217 377 6.7 14.5 7 '27. 1 19.0 20. 8 60 15.0 I 41. 4 70 38.0 4 22.7 55 36.0 30. 0 58 48.0 4 20. 0 53 38.0 6 2 61. 5 B 19.0 VI {A 14.0 47.7

1 Tube increases 78.0 cm. (before) to 81.6 cm. after rotary swaging.

1 About 12 inches long initially.

3 34.5 inches long at end of swaging operation.

4 40 inches long initially.

5 43 inches long initially.

In cm. of Hg. where marked: otherwise in cm. of 11 0.

"Air flow cubic feet per hour per tube; effective length about 12inches.

EXAMPLE VII A filter tube was fabricated from 325 mesh 40 micron)fraction of 316 L stainless steel as described above and about 150 cm.long. The bubble point of this tube was 10 cm. of Hg. After swaging,'thebubble point of the tube was 16 cm. Hg. A profilometer was used todetermine the surface roughness. Ignoring the holes or pores, theresulting surface was about 4 micro inches /2 thereby eliminating theneed for a subsequent step to open pores. Because bubble point is ameasure of the largest or maximum pores an increase means reduction insize of the largest pores but not necessarily proportional dimensionalchanges in all pores. The porosity is lower at higher bubble points anddoubling thus corresponds approximately to halving overall porosity.

The finer porosity tube when swaged on a mandrel loses nearly all of;itssurface porosity (greater than RMS, yet the porosity was open anduniform.

EXAMPLE VIII A filter tube about 150 cm. long with a bubble point of12.5 cm. H O was swaged and the result was a tube with a bubble point of15.8 cm. H O. The ultimate tensile strength went from an average of13,200 p.s.i. before swaging to 20,500 p.s.i. after swaging. The densitychanged from 53% to 61.5% of theoretical, yet porosity remained open anduniform.

EXAMPLE IX The diameter of a piece of open-ended porous tubing 52 incheslong with a bubble point of 16 cm. Hg was measured at 2 inch intervalsalong the tube. The total deviation from nominal along the tube lengthwas .0005 inch. A single point measured .0005 inch less than the rest ofthe tube. Porosity remained open and uniform. The diameter after swagingwas .37475 $00025 inch.

EXAMPLE X This example was performed in part according to US. Pat. No.3,313,621 with swaging on a mandrel. Porous stainless tubing in about 12inch lengths of borth coarse and fine micronic ratings were swaged andwithout mandrels. Table II summarizes the results.

cm. Hg pressure) and must be etched to be reopened. Those pores thatstill remain open are erratic as to their position. They are too few topermit of any reasonable gas flow.

EXAMPLE )6 Tubes with coarse and fine micronic ratings as in Example Xare reduced in diameter in a different embodiment of the process of theinvention by rotating a section of the tube in a lathe so that 25 mm. (1inch) diameter 0.7 cm. (0.25 inch) wide steel idler rollers are forcedagainst it simultaneously and with about equal force therebycentripetally applying mechanical force in three longitudinal zones.The. rolls are mounted on the tool post which is mechanically traversedso that the rolls are moved slowly axially along the rotating tube.Reduction in porosity is effected with no visible smearing of thesurface due to tangential forces.

What is claimed is:

1. A porous sintered metallic tube having a rough sintered inner surfaceand a mechanically worked dimensioned outer surface.

TABLE II Outer Inner Percent Percent Bubble Pressure diameter diameterwall retheoretical point drop 1 (inch) (inch) duction density (cm. H O)(cm. H 0) Coarse material:

As sintered 435 294 53 16 4 Swaged without a mandrel. 377 238 1. 4 6421. 5 8 7 Swaged on a mandrel 382 284 31. 0 70 38 42. 7 Fine material:

As sintered 454 278 70 1 5 54 Swaged without a mandrel 376 182 +11 80 210 623 Swaged on a mandrel 385 250 24 93 2 50 (3 1 gressfire dropmeasured at an air flow of cubic feet per hour. I m. g. I Substantiallyimpermeable to gas flow.

It is evident that swaging on a mandrel results in 25- 50% wallreduction and drastically reduces the air flow through the tube. Swagingwithout the use of a mandrel has the effect of only decreasing the airflow to some extent while still sizing the outside diameter andretaining an open porosity as measured by bubble point of about 2. Aporous tube according to claim 1 composed of stainless steel.

3. A porous tube according to claim 2 having one end closed.

(References on following page) 8 References Cited 697,291 9/1953 GreatBritain 75222 707,512 4/1954 Great Britain 75 222 UNITED STATES PATENTS714,560 9/1954 Great Britain 75-222 213;; gfi 23 717,034 10/1954 GreatBrita n 75-222 66 Holtsclaw 2 X 5 832,317 4/1960 Great Bntam 75222 g?BENJAMIN R. PADGETT, Primary Examiner 1 t B2326 R. E. ASSlStaDt EXammerFOREIGN PATENTS 10 U.S. Cl. X.R. 7/1956 Canada 7s 222 203, 222

